This is a phase-II/III to compare the efficacy and toxicity of 2',3' dideoxyinosine (ddI) and zidovudine (ZVD) in patients with AIDS or advanced ARC who have received AZT for 12 months or longer. Patients will be assigned to one of two treatment arms under a double-blind, randomly allocated experimental design. ddI will be administered orally at a dose of 375 mg twice daily, and ZVD orally at a dose of 200 mg every 4 hours.